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Its time for a carb conversion - but dont start judging me, this is going to be for track use and I have been talked into it as a viable alternative to spending big money on a computer that will run this engine better than a chipped 870 !
While I was researching the subject I noticed there was a lack of good info on the subject (in a start to finish thread), apart from "yeah you just rip the injection off and bung the carb on - dead simple !" Well Im already down the road somewhat and guess what, as is c4 its not that simple (depending on your choices), so I thought I would post a thread with some photo's and issues Ive struck for the people who are interested in the topic.
First thing - a carby ! choice is a 735 Quick Fuel secondary - my car is an auto, word from the wise is dont use a double pumper on an auto car.
Next thing is the computer controlled distributor wont work with this setup - will need a 'normal' HEI distributor, so an MSD is the choice there.
this is also a street reg car so keep that in mind.
As always feedback from others that have gone this route is welcolme !
As an explanation as to where Im starting from, I already have a single plane intake EFI (using a Procomp Hurricane manifold) that has a custom elbow on top, this setup JUST fits under the hood. A carb on top of this intake will not fit under the hood, so I will be cutting a hole and putting a scoop on because I cant be bothered fighting against it !
Im sure others that have fitted under the hood can chime in with recommendations for intakes that do fit the c4, as I know that is a big consideration for some contemplating the carb conversion.
first step to get some carb room is cutting the hole in the hood - in between the underside hood supports.
What are you doing with the single plane and plenum?
What size motor do you have?
I will be using the single plane with the carb -all the injection stuff is being unbolted and I will be setting it up so I can make the change back process straight forward.
I bought my 84 already converted. Edelbrock Performer RPM, Edelbrock 600, low sitting air cleaner. Looks like some wiring changes behind breadbox. Distributor was changed also.
Stock hood.
I carbed my C4 years ago. I was able to fit a Holley 750 vacuum carb uner the stock hood with a Weiand dual plane and a low profile air cleaner. I had a problem with fuel percolation after shutdown. I an mow running a miniram
There is no doubt you can get them under the stock hood, the research I did on the subject seemed to show that heat soak was an issue trying to keep a carb under the stock hood, not sure if that is more of a race or street issue ?
I see that Quick Fuel sell insulator plates to protect the bowls from heat.
I cant be bothered fighting that fight so there is a hole and scoop on mine now.
test fit of 14" x 3" air cleaner thru the hole, you can see how much offset is in the c4 engine !
I have gone with a 70mm high reverse cowl scoop to get the windscreen air in but not too high, still need to see the apexes !
first consideration with carb is PCV and brake booster into the intake.
the Quick Fuel carb has only 1 x 3/8 pipe, and it is recommended only to be used for PCV. If you do a search on the topic you will find plenty of lively arguements about using a T piece and running both off the one line. If you check the fact sheets from the manufacturers they say dont do it - I have asked knowledgeable people on the subject and they say dont do it. But there are plenty of people on different forums that will say they have done it and it doesnt adversely affect anything. Each to their own I guess - but it is a consideration for the carb conversion on these cars.
After having the scoop on I know that I have 1 more inch left - so I will kill 2 birds with one stone, and use a 1" super sucker style spacer. I have tapped it at the rear and screwed on a -6 fitting to run the brake booster line.
For those that dont know the spacers generally give more horsepower (on single plane intakes), but is usually dependant on the combo. But as with all things mechanical what you get at one end you lose at the other, the super sucker style spacers create a velocity to get back some low and mid range that a spacer can lose.
If your going carb under the hood, you will have to hope that your carb has 2 x 3/8 fittings, find another place to take it from (some intakes have tapped holes and fittings in them) or T piece .
Here is a pic from the rear, left line is PCV out the rear of the carb, and right line is the brake booster using a push on -6 fitting out of the spacer.
First major problem - even with the scoop on the air filter is now hitting it.
I wasnt real sure about using one of these filter tops - I measured the lid and its 12 mm or about 1/2" thick, so I have changed over to a Moroso drop base filter, the stamped lids have a curve in them and are only about 2 mm thick = now it fits under the scoop !
next step now that the old efi stuff is out of the way, change the regulator. The efi needs around 34 to 55 psi to run (depending on what youve got) and the carb needs 6.5 psi, so you need a carb regulator. I have chosen a Mallory unit that is designed to take the feed of a high volume efi pump and regulate it down for a carb.
in the pic you can see the carb on and fuel lines and regulator being set up. I already had my fuel system coming up the firewall so I have adapted from there.
I have taken out the Aeromotive 340 pump and put the Walbro 255 back in to help the regulator out a bit.
Question, Are you using a lockup kit for your transmission and what did you do about the charcoal canister.
yes, the torque converter lockup becomes an issue when you go to carb - usually the computer uses it inputs to lock up the converter at a certain speed and throttle position input.
due to what Im doing with the car Im not too worried about the lockup at this stage.
there are a couple of ways to do it - piggyback the wires A and F in the ALDL plug to a toggle switch, then you can switch it on and off, but if you have to remember to switch it off before you come to a stop. Usually there is a wire on the brake switch to de activate it - maybe someone who has done this method on the c4 can advise ?
or you can buy a lockup kit,
The charcoal cannister was disconnected long ago on mine, its still in the light well with wiring etc, but no tubing. Never set an engine light with it disconnected.
05-14-2017, 08:22 AM
C4 Carb conversion
